Method and apparatus for detecting leaks in the ullage of a liquid storage tank

Abstract

Apparatus and method for detecting leaks within underground or aboveground tanks and their associated piping whether the tank and/or piping is partially filled or empty. The apparatus is partially insulated to minimize temperature effects when injecting a test gas into the tank to be tested. The tank is pressurized to a fill pressure by injecting (bubbling) the test gas through any liquid in the tank. Once stabilized the pressure is reduced to a test pressure while the gas injection continues. The flow rate of gas into the tank is monitored by a flow meter able to measure small flows while the pressure is held constant by a pressure regulator having a high accuracy and the temperature of the ullage is monitored. A tight or leaking tank is determined by evaluating the flow rate of constant pressure against reference criteria which when met identify a tight tank and when not met identify a leaking tank under EPA guidelines. Temperature, deflection, and other effects on leak rate detection are compensated for by prescribing waiting periods, filling times, and multiple and repeat threshold exceedance criteria. Flow rate, temperature and pressure can be reached manually or fed to a suitable computer or chart recorders for permanent records and/or automated testing.

What is claimed is: 
     
       1. An apparatus for detecting and measuring leaks in a closed partially filled or empty tank comprising: pressure control means for regulating pressure of gas supplied at an inlet of said pressure control means to a pre-set value and including means for monitoring flow of gas through said pressure control means to an outlet of said pressure control means;   test cap means for sealing an opening of the tank, said test cap means having an opening therethrough,   first connecting means for connecting said outlet of said pressure control means to a first side of said opening of said test cap means, said first connecting means having insulation substantially covering said first connecting means; and   injection means having a first end connected to a second side of said opening in said test cap means extending into and to a point substantially adjacent to the bottom of said partially filled or empty tank, said injection means having a first aperture at or near said first end of said injection means, said injection means having a set of one or more second apertures at or near a second end of said injection means, for injecting said gas into the tank through said first and second apertures, wherein a cross sectional area of said first aperture is substantially less than a cross sectional area of said set of second apertures.   
     
     
       2. An apparatus in claim 1 further comprising gas source means for supplying gas from a gas source through a second connecting means to an inlet of said pressure control means. 
     
     
       3. An apparatus as in claim 2, wherein insulation substantially covers said second connecting means. 
     
     
       4. An apparatus as in claim 1, wherein said injection means is a flexible tube. 
     
     
       5. An apparatus as in claim 4, wherein said flexible tube is attached to a ballast weight at said second end. 
     
     
       6. An apparatus as in claim 1, wherein said test cap means includes a temperature probe configured to measure temperature inside the tank wherein said temperature probe is disposed at approximately the center of the ullage of said tank. 
     
     
       7. An apparatus as in claim 1, further comprising a fill regulator providing a second flow path between the gas source and said first connecting means when a fill/test valve included in said pressure control means is turned to a fill position. 
     
     
       8. An apparatus as in claim 1, wherein a plurality of pressure control means are provided, each individually connected to a separate tank. 
     
     
       9. An apparatus as in claim 1, wherein said pressure control means includes a pressure gauge which senses the pressure at said outlet of said pressure control means. 
     
     
       10. A method contributing to the detection and measurement of leaks in a closed tank comprising the steps of: determining the tank's liquid level history for a first predetermined period of time prior to the test;   deciding whether to wait for a second predetermined period of time if there has been a substantial change in the tank's liquid volume;   determining the tank's dimensions;   determining the level of liquid in the tank;   calculating the ullage volume of the tank;   calculating a maximum tank fill pressure so that the pressure at the bottom of the tank does not exceed a maximum tank allowable pressure;   pressurizing the tank from a gas source to the maximum tank fill pressure;   holding the tank at the maximum tank fill pressure for a third predetermined time until said test pressure is stable to within a first predetermined tolerance;   recording the tank ullage temperature;   venting the tank to arrive at a test pressure;   monitoring the flow rate of gas into the tank at the test pressure for a fourth predetermined time while maintaining the pressure at said test pressure within a second predetermined tolerance;   identifying a first stable flow rate; and   comparing said first stable flow rate with a reference criteria to determine whether, this, a first test cycle receives a passing grade because no unacceptable leak was detected.   
     
     
       11. A method for detecting and measuring leaks in an underground tank and piping partially filled with liquid comprising: measuring the flow rate of a gas into the tank over a predetermined interval while simultaneously maintaining constant pressure in the tank and determining whether a leak has occurred based on a plurality of test cycles as described in claim 10, wherein a second cycle of said plurality immediately follows said first test cycle whose results are compared against a set of acceptance criteria.   
     
     
       12. The method of claim 11, wherein an insulated hose is connected from a control panel to the tank for injecting said gas during the step of measuring said flow rate. 
     
     
       13. A method as in claim 10, wherein said step of calculating said maximum tank fill pressure includes: subtracting from the maximum tank allowable operating pressure, the pressure at the bottom of the tank due only to the liquid level inside the tank, arriving at a remainder; and   determining whether the ground water level outside the tank is higher than the bottom of the tank, and when the ground water level outside the tank is higher than the bottom of the tank;   determining the ground water pressure at the bottom of the tank; and   adding to said remainder said ground water pressure.   
     
     
       14. A method as in claim 10, wherein said step of comparing said first stable flow rate with said reference criteria includes performing and monitoring a set of at least two test cycles which repeatedly provide data that show whether each test cycle receives a passing grade because no acceptable leak was detected. 
     
     
       15. A method as in claim 10, wherein the step of recording the tank temperature includes verifying that the tank ullage temperature has stayed within a predetermined set of prescribed temperature limits throughout a series of test cycles to establish a valid test. 
     
     
       16. A method contributing to the detection and measurement of leaks in a closed tank as in claim 10, further comprising the steps of: performing a second test cycle by: recording the tank ullage temperature;   monitoring the flow rate of gas into the tank at the test pressure for said fourth predetermined time while maintaining the pressure at said test pressure within said second predetermined tolerance;   identifying a second stable flow rate; and   comparing said second stable flow rate with said reference criteria to determine whether, said second test cycle receives a passing grade because no unacceptable leak was detected.     
     
     
       17. A method contributing to the detection and measurement of leaks in a closed tank as in claim 16, further comprising the steps of: determining whether said first test cycle and said second test cycle have both received a passing grade because no acceptable leak was detected and if they have then reporting that the tank meets the leakage criteria.   
     
     
       18. A device to be used in conjunction with an apparatus for detecting leaks in a closed tank comprising: a gas injection tube having a first end connected to an aperture extending through a test cap means dealing an opening of said tank, said tube extending into and to a point substantially adjacent to the bottom of said closed tank, said injection tube having a first aperture at or near said first end of said injection tube, said injection means having a set of one or more second apertures at or near a second end of said injection means, for injecting said gas into the tank through said first and second apertures, wherein a cross sectional area of said first aperture is substantially less than a cross sectional area of said set of second apertures.